Rotary heat exchangers of the class under consideration are of widespread and important application. They are useful, for example in recovering thermal energy from the contaminated exhaust effluents of laundry dryers, grain dryers, asphalt aggregate mixers, and the various processing units to be found in the textile, food and fiberboard manufacturing industries. They rely for heat exchange function upon the inclusion in their structures of a plurality of Perkins tubes.
It is the general purpose of the present invention to provide a novel heat exchanger of the described class which is of simple, relatively inexpensive construction but of greatly improved effectiveness. As a consequence, its use in the various applications to which it is suited has the potential of resulting in significant savings of heat energy, and hence of operating costs.
Briefly stated, the presently described rotary heat exchanger includes in its assembly a rotor traversing an evaporation chamber and a condensation chamber. A plurality of Perkins tubes having evaporation sections and condensation sections is mounted on the rotor. The evaporation sections of the Perkins tubes extend into the evaporation chamber and the condensation sections extend into the condensation chamber.
It has not been found possible to improve the effectiveness of rotary heat exchangers by employing capillary means to redistribute the working fluid circumferentially in the evaporation section. This is because the high force fields created by rotation strongly suppress capillarity, thereby rendering this mechanism ineffective.
To circumvent the disadvantage posed by the lack of capillarity, the present invention is predicated on the discovery that by the simple expedient of providing Perkins tubes of the above construction wherein the tube evaporation sections are displaced radially outwardly from the condensation sections with reference to the axis of rotation of the rotor, and using in the Perkins tubes a working fluid in amount sufficient to optimally occupy the evaporation sections with fluid while substantially eliminating the presence of fluid from the condenser sections, the efficiency of the evaporation cycle of the former and the condensation cycle of the latter is increased to a significant extent. This results in important energy and, consequently, economic savings during operation of the heat exchanger.
Without commitment to a particular heat transfer theory, it is known that this result stems from improving the efficiency with which the working fluid contained in the Perkins tubes is vaporized in the evaporation sections of the tubes and condensed in the condensation sections thereof. The entire inner surface area of the evaporation section of each Perkins tube is heated by the exhaust gas, and this entire inner surface is capable of heating and vaporizing the working fluid. This optimum heat transfer condition can only obtain if the working fluid is in direct contact with the entire inner surface. However, because space must be provided for vapor flow, the working fluid cannot completely occupy and thereby completely contact the entire inner surface of the evaporation section. It is readily apparent that an optimum heat transfer and vapor flow area condition exists wherein the disposition of the working fluid is such as to maximize the inner surface area in contact with the working fluid and, simultaneously, provide the required vapor flow space.
The entire inner surface area of the condensation section of each Perkins tube is cooled by the supply gas, and this entire inner surface is capable of cooling and condensing the working fluid vapor. The optimum heat transfer condition can only obtain if the working fluid vapor is in direct contact with the entire inner surface. This condition obtains when the condensation section of each Perkins tube is substantially free of working fluid. The overall result is a significantly improved efficiency of the heat exchanger.